The present invention relates generally to thin film inductors and the articles comprising the structure therefor.
With the increasing trend of miniaturization of electrical circuits, it is expected that thin film inductors will find applications in AC circuits such as those for on-chip power management and signal processing for wireless communications products. For example, inductors intended for power management will be required to operate in the 10 MHz region, have relatively large inductance and be able to handle large driving currents. For wireless communication applications, it is anticipated that ultra high frequency ( greater than 1 GHz) inductors will be utilized, where inductance and driving currents that are required are comparatively small relative to power management applications.
Currently, typical bulk inductors are made by wrapping conducting coils around a magnetic torroid. Often an air gap is put into the torroid to control the magnetic properties. The effect of the air gap is to manipulate the internal magnetic field (Hi) such that
Hi=Haxe2x88x92NM
where Ha, is the applied field, M is the magnetization of the ferromagnetic material and N is a demagnetizing constant which is dependent on the geometry of the gapped inductor. Because a structure will try and magnetize itself such that its internal magnetic field is zero, the magnetization increases linearly with an applied magnetic field such that the shape of a corresponding hysteresis loop becomes sheared with respect to an ungapped structure. This slanting of the hysteresis loop is also responsible for maximizing the energy storage of the inductorxe2x80x94since Estored=xc2xdLImax2, where L is the inductance and Imax is the maximum current of the coils. In addition, since Imax is proportional to the maximum magnetic field (Hmax) and L proportional to the permeability       (          μ      =                        Δ          ⁢                      xe2x80x83                    ⁢          B                          Δ          ⁢                      xe2x80x83                    ⁢          H                      )    ,
it can be seen that optimization of the energy storage occurs when Hmax is just below Hsat.
The geometry of planar inductors causes some magnetic effects which are dissimilar to magnetic effects found in bulk inductors. These differences must be considered when designing a planar inductor for maximum efficiency in the application of interest. For example, due to the shape anisotropy of thin films, magnetization typically is confined in the plane of the magnetic film, essentially causing a two dimensional magnetization reversal. Soft magnetic films which would be used for planar inductors typically have an additional in-plane uniaxial anisotropy energy, where the magnetization is of low energy when along the xe2x80x98easy axisxe2x80x99 and high energy when along the xe2x80x98hard axisxe2x80x99. Since this energy which controls the magnetization reversal process is typically uniaxial, the easy axis is perpendicular to the hard axis. This causes the magnetization reversal to predominantly occur by magnetic domain wall motion when a field is applied parallel to the easy axis and by magnetization rotation when a field is parallel to the hard axis. Magnetization rotation produces a linear hysteresis loop with a saturating field equal to the anisotropy field (Hk=2Ku/Ms) where Ku is the uniaxial anisotropy constant. As is known, this uniaxial anisotropy can be produced in ferromagnetic films through different mechanisms such as uniaxial stress (magnetoelastic energy), magnetically induced anisotropy (an external magnetic field applied to the film during deposition or annealing), crystal anisotropy (when an in-plane crystallographic texture is present), tilted columnar microstructure (a micro magnetostatic energy) or as a result of the shape of a patterned magnetic structure (a macro magnetostatic energy). Because of the resulting linear hysteresis loop of magnetization rotation, the effect of increasing the uniaxial anisotropy is analogous to the effect of increasing the gap size in bulk torroids.
For ultra high frequency inductor applications it is also advantageous to have a uniaxial anisotropy in the ferromagnetic layers. It is also beneficial to operate the magnetization reversal by rotation mechanisms because rotation typically has higher ferromagnetic resonance frequencies and lower losses than domain wall motion mechanisms. This is especially important at ultra high frequencies. The ferromagnetic resonance frequency can be used to calculate a cut-off frequency for the usefulness of a magnetic inductor. The resonance frequency for magnetization rotation of a thin ferromagnetic film can be calculated to be   fres  =            γ              2        ⁢        π              ⁢                  Hk4        ⁢                  xe2x80x83                ⁢        π        ⁢                  xe2x80x83                ⁢        Ms            
where xcex3 is the gyromagnetic constant.       (                            γ                      2            ⁢            π                          ~        2.8            ⁢              xe2x80x83            ⁢              MHz        /        Oe              )    .
Theoretically the maximum permeability of rotation is given by 4xcfx80Ms/Hk as a first approximation. Again, the importance of controlling this anisotropy for the application and frequency of interest can be seen, since a large Hk increases the resonance frequency. At the same time, however, large Hk also decreases the permeability.
The prior work on the design of thin film inductors has focused primarily on the shape of the conducting coil. For example, Kawabe et al., IEEE Trans. Mag. V20, #5, p. 1804-1806 (1984) describes planar coils with hoop type, spiral type and meander type configurations. Sato et al, IEEE Trans. Mag. V30 #2,p.217-223 (1994) describes a double rectangular spiral coil. Most of these inductors utilize rectangular or square shaped magnetic films above and/or below the plane of the conducting coils. In their analysis Kawabe et al. assume a constant permeability and do not take into consideration an anisotropy in the magnetic layer. However, Sato et al. and Yamaguchi et al., presented at MMM Miami, Fla., November, 1998 treat a more realistic model which takes into account magnetically induced anisotropy produced by external magnetic fields which occur during processing or subsequent annealing.
An example of a prior art type of configuration for a planar inductor 10 is shown in FIG. 1. As shown, the planar inductor 10 comprises a top magnetic layer 12 and bottom magnetic layer 14 including, for example, magnetic film conductor coils 16 sandwiched between the two layers. Referring to FIG. 2, it can be seen that region A has an applied field parallel to the easy axis. Region B has the applied field parallel to the hard axis. As would be understood by a person skilled in the art, this means that region A will operate by domain wall motion mechanisms which are not beneficial for high frequency applications. This is because domain wall motion has higher losses and lower ferromagnetic resonating frequencies than rotation mechanisms. Region B would operate by magnetization rotation mechanisms which are the desired mechanism of magnetization reversal for high frequencies. Some prior art designs manipulate the dimensions of the magnetic layer in order to eliminate region A and as a result the complete coil is not utilized.
Another type of inductor described in prior art literature is the stripe inductor which involves a conductor sandwiched between two magnetic layers in the form of a stripe. For these inductors the magnetic material either completely encloses each segment of conductor or has the same width as each segment of conductor. The stripe inductor has been proposed for UHF applications and will contain a shape anisotropy which must be considered for device design as will be discussed. Based on the above, it can be seen that a need exists in the design of planar inductors which better takes into consideration the existence of the anisotropies of the magnetic layers.
The present invention is a planar spiral inductor including a top magnetic layer a bottom magnetic layer and a plurality of conductive coils disposed between the top magnetic layer and the bottom magnetic layer. A significant difference from prior art is that the top and bottom magnetic layers have their centers effectively cut out using lithographic techniques or other techniques to frame the core of the conductive spirals. An advantage of this structure over the prior art is that when magnetic anisotropies other than shape are kept small, the magnetic configuration will produce a magnetostatic shape anisotropy such that the easy axis (low energy direction of magnetization) lies parallel to the legs of a rectangular frame or the circumference of a circular frame. During operation of the inductor, the field produced by the coils flows in a radial direction and will be perpendicular to the easy axis direction thereby causing magnetization reversal to occur by rotation while advantageously utilizing the full structure in this mode.